webkitpy: prune unused code in scm and make tests pass

This patch removes a bunch of no-longer needed code from the scm modules in webkitpy.common.checkout.scm. This is safe because we no longer use any of this logic to push commits to the server (though we still support local commits), and so we can shrink the API for the base SCM interface substantially and delete a lot of tests.

This patch also updates scm_unittest so that all of the remaining tests pass. The tests are still slow and not run by default in test-webkitpy.

This patch depends on https://codereview.chromium.org/27055003/ .

BUG=
R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/27045007

git-svn-id: svn://svn.chromium.org/blink/trunk@159600 bbb929c8-8fbe-4397-9dbb-9b2b20218538